High-purity styrenes usable for industries have been conventionally produced by separating and purifying a mixture generated from reaction by distillation. Styrenes, such as styrene, a substituted styrene, divinylbenzene, etc. have property of easily being polymerized, and thus, distillation is conventionally effected in the presence of a polymerization inhibiting agent.
A wide variety of compounds have been investigated for composing an agent of inhibiting the polymerization of the styrenes. However, of those conventional agents of inhibiting the polymerization of the styrenes available today, only nitrophenols and nitrosophenols have proved to be durable for industrial use in the distillation process. When sulfur is made available for example, environmental contamination arising from the burning of distilled residue is the critical problem. On the other hand, substituted alkylphenols, such as p-tertiarybutyl catechol, etc. have not enough ability to inhibit the polymerization of the styrenes at the distillation temperature when being used alone.
In addition, nitrophenols, such as 2,4-dinitrophenol, 2-methyl-4,6-dinitrophenol, 4-methyl-2,6-dinitrophenol, etc. have also been investigated. Nevertheless, any of these has mild activity to serve as the agent of inhibiting the polymerization of the styrenes, but instead, these respectively function as the polymerization retarding agent, and thus, any of these must be used with fairly high concentration. Considering toxicity incurring to human body, the nitrophenols cannot be determined as an ideal agent suited for inhibiting polymerization of the styrenes.
On the other hand, nitrosophenols have lower toxicity than that of the nitrophenols. Furthermore, according to JP-B-55-37974 (the term "JP-B" as used herein means an "examined Japanese patent publication"), in terms of the activity for inhibiting the polymerization of the styrenes, due to high reactivity of the nitrosophenols against an alkyl radical which causes the styrenes to generate thermal polymerization reaction and also due to high activity in inhibiting the polymerization, the nitrosophenols can effectively suppress the thermal polymerization of the styrenes.
Nevertheless, due to high reactivity of the nitrosophenols against the thermally generated alkyl radical, i.e., due to high activity for inhibiting the polymerization of the styrenes, after fully inhibiting the polymerization of the styrenes, a certain polymerization similar to the thermal polymerization in the absence of the inhibiting agent may be generated, and thus, in order to use it on the industrial basis, extreme precision is required for controlling the concentration of the polymerization inhibiting agent and the operating factors, such as temperature, pressure, etc. related to the distillation system.